<html>
<head lang="en">
    <meta charset="UTF-8">
</head>
<body>
<?php
function spamcheck($field)
{
    //filter_var() sanitizes the e-mail
    //address using FILTER_SANITIZE_EMAIL
    $field = filter_var($field, FILTER_SANITIZE_EMAIL);

    //filter_var() validates the e-mail
    //address using FILTER_VALIDATE_EMAIL
    if (filter_var($field, FILTER_VALIDATE_EMAIL)) {
        return TRUE;
    } else {
        return FALSE;
    }
}

if (isset($_REQUEST['email'])) {//if "email" is filled out, proceed

    //check if the email address is invalid
    $mailcheck = spamcheck($_REQUEST['email']);
    if ($mailcheck == FALSE) {
        echo "Invalid input";
    } else {//send email
        $email = $_REQUEST['email'];
        $subject = $_REQUEST['subject'];
        $message = $_REQUEST['message'];
        mail("someone@example.com", "Subject: $subject",
            $message, "From: $email");
        echo "Thank you for using our mail form";
    }
} else {//if "email" is not filled out, display the form
    echo "<form method='post' action='email.php'>
  Email: <input name='email' type='text' /><br />
  Subject: <input name='subject' type='text' /><br />
  Message:<br />
  <textarea name='message' rows='15' cols='40'>
  </textarea><br />
  <input type='submit' />
  </form>";
}
?>

</body>
</html>